1. Field of the Invention
The present invention relates to a fixing apparatus which fixes a toner image by heating the toner image formed on a sheet, and an image forming apparatus including the same.
2. Description of the Related Art
A copying machine, a printer, or a facsimile, which performs image formation by an electrophotographic system, an electrostatic recording system, or the like, or an image forming apparatus having multiple functions thereof is mounted with a fixing apparatus which fixes a toner image formed on a sheet.
As a heating portion which heats a fixing rotating member of a fixing apparatus, there has been proposed an electromagnetic induction heating portion which generates heat by Joule heat by generating an eddy current in an induction heating member provided in a fixing rotating member by an exciting coil. When small-size sheets are heated in succession, the following problems occur. That is, in a region (sheet passing region) where a sheet contacts a surface of a fixing rotating member, the sheet is conveyed while heat is being transferred to the sheet. In contrast, in a region (sheet non-passing region) where the sheet does not contact the surface of the fixing rotating member, no heat is transferred to the sheet. Thus, heat is accumulated in the fixing rotating member, causing an excessive temperature rise. Therefore, a great temperature difference occurs between the sheet passing region and the sheet non-passing region in the fixing rotating member.
Japanese Patent Laid-Open No. 2006-267180 discloses a technique which adjusts a temperature of a sheet non-passing region in a fixing rotating member by inserting or removing a magnetic shielding plate in a gap portion between a fixing rotating member having an electromagnetic induction heat generating layer and an exciting coil which generates a magnetic flux. That is, when there is a risk of the excessive temperature rise in the sheet non-passing region by conveying a small-size sheet, the arrival of the magnetic flux at the fixing rotating member is blocked by inserting the magnetic shielding plate into the gap portion, and the heat generation of the fixing rotating member in the sheet non-passing region is suppressed.
Meanwhile, when conveying a large-size sheet, the magnetic shielding plate is moved to the outside of the gap portion, and the entire area of the fixing rotating member generates heat. The magnetic shielding plate is made of a material having high permeability and high electrical resistance (for example, ferrite). Since the material having high electrical resistance is used, a current value becomes small, and thus, heat generation of the magnetic shielding plate itself is suppressed.
As disclosed in Japanese Patent Laid-Open No. 2006-267180, in the configuration including the movable magnetic shielding plate, when the magnetic shielding plate is disposed at a position which is an upstream side in a sheet conveying direction rather than a fixing nip portion of a fixing apparatus and is in vicinity of the fixing nip portion, a space is required for disposing the magnetic shielding plate at a position which is an upstream side of the fixing nip portion and is in the vicinity of the fixing nip portion.
Meanwhile, in many cases, the sheet being conveyed to the fixing nip portion is curled. Therefore, a sheet conveying guide is disposed for certainly conveying the sheet to the fixing nip portion. Even though the magnetic shielding plate intends to be disposed at the position which is the upstream side in the sheet conveying direction rather than the fixing nip portion and is in the vicinity of the fixing nip portion, the magnetic shielding plate may not be disposed due to the sheet conveying guide, or the shape of the magnetic shielding plate is restricted. Thus, the magnetic shielding function may not be sufficiently exhibited.
The present invention is for solving the above-described problems, and in a fixing apparatus including a magnetic flux shielding portion which shields a magnetic flux generated from an electromagnetic induction heating portion, it is desirable to provide a fixing apparatus which can have both functions of a magnetic flux shielding portion and a sheet conveying guide.